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.Dd June 20, 2021
.Dt KENV 2
.Os
.Sh NAME
.Nm kenv
.Nd kernel environment
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In kenv.h
.Ft int
.Fn kenv "int action" "const char *name" "char *value" "int len"
.Sh DESCRIPTION
The
.Fn kenv
system call manipulates kernel environment variables.
It supports the well known userland actions of getting, setting and unsetting
environment variables, as well as the ability to dump all of the entries in
the kernel environment.
.Pp
The
.Fa action
argument can be one of the following:
.Bl -tag -width ".Dv KENV_DUMP_LOADER"
.It Dv KENV_GET
Get the
.Fa value
of the variable with the given
.Fa name .
The size of the
.Fa value
buffer is given by
.Fa len ,
which should be at least
.Dv KENV_MVALLEN
+ 1 bytes to avoid truncation and to ensure NUL termination.
.It Dv KENV_SET
Set or add a variable.
The
.Fa name
and
.Fa value
are limited to
.Dv KENV_MNAMELEN
and
.Dv KENV_MVALLEN
characters, respectively
.Pq not including the NUL terminator.
The
.Fa len
argument indicates the length of the
.Fa value
and must include the NUL terminator.
This option is only available to the superuser.
.It Dv KENV_UNSET
Unset the variable with the given
.Fa name .
The
.Fa value
and
.Fa len
arguments are ignored.
This option is only available to the superuser.
.It Dv KENV_DUMP
Dump as much of the dynamic kernel environment as will fit in
.Fa value ,
whose size is given in
.Fa len .
If
.Fa value
is
.Dv NULL ,
.Fn kenv
will return the number of bytes required to copy out the entire environment.
The
.Fa name
is ignored.
.It Dv KENV_DUMP_LOADER
Dump the static environment provided by
.Xr loader 8 ,
with semantics identical to
.Dv KENV_DUMP .
Duplicate and malformed variables originally present in this environment are
discarded by the kernel and will not appear in the output.
.It Dv KENV_DUMP_STATIC
Dump the static environment defined by the kernel
.Xr config 5 .
The semantics are identical to
.Dv KENV_DUMP_LOADER .
.El
.Sh RETURN VALUES
The
.Fn kenv
system call returns 0 if successful in the case of
.Dv KENV_SET
and
.Dv KENV_UNSET ,
and the number of bytes copied into
.Fa value
in the case of
.Dv KENV_DUMP
and
.Dv KENV_GET .
If an error occurs, a value of \-1 is returned and
the global variable
.Va errno
is set to indicate the error.
.Sh ERRORS
The
.Fn kenv
system call
will fail if:
.Bl -tag -width Er
.It Bq Er EINVAL
The
.Fa action
argument
is not a valid option, or the length of the
.Fa value
is less than 1 for a
.Dv KENV_SET .
.It Bq Er ENOENT
No value could be found for
.Fa name
for a
.Dv KENV_GET
or
.Dv KENV_UNSET .
.It Bq Er ENOENT
The requested environment is not available for a
.Dv KENV_DUMP_LOADER
or
.Dv KENV_DUMP_STATIC .
The kernel is configured to destroy these environments by default.
.It Bq Er EPERM
A user other than the superuser attempted to set or unset a kernel
environment variable.
.It Bq Er EFAULT
A bad address was encountered while attempting to copy in user arguments
or copy out value(s).
.It Bq Er ENAMETOOLONG
The
.Fa name
or the
.Fa value
is longer than
.Dv KENV_MNAMELEN
or
.Dv KENV_MVALLEN
characters, respectively, or
.Fa len
did not include the NUL terminator for a
.Dv KENV_SET .
.El
.Sh SEE ALSO
.Xr kenv 1
.Sh AUTHORS
.An -nosplit
This manual page was written by
.An Chad David Aq Mt davidc@FreeBSD.org .
.Pp
The
.Fn kenv
system call was written by
.An Maxime Henrion Aq Mt mux@FreeBSD.org .
